[How to] Air flow

Your PC case should consist of at least 3 fans( 1 intake, 2 out take) we are gonna use my PC as a reference. (pics are crappy so bare with me.)

your front of your case should posses the intake fan this will suck in air to be properly circulated within your case. later to be distribute outwards via the exhaust fans.Overall direction you wanna achieve.
http://img230.exs.cx/img230/2493/picture0196lh.jpg
Above you can see what your gonna need to achieve.

as stated earlier your intake should consist of at LEAST 1 fan, I have 2 but you get the idea.
http://img185.exs.cx/img185/259/picture0215yj.jpg
When installing this fan, remove the front panel by way of the 4 screws located behind the face plate itself. this will enable easy install and a hassle free experience. (Do not be in a rush to build your system )
secondly, as stated earlier as well your gonna want at least 2 (depending on the size of the case, fulltower cases will let you have 2 outtake fans. Besides the PSU fan, like i have pictured )
http://img177.exs.cx/img177/5225/picture0207cc.jpg
As indicated by the arrows, this is the direction of the air being removed by these fans.( it has to go somewhere it cannot remain in your system, or your components will slowly bake over due time.)

side intake
As an added bonus( mostly depending on the type of rig) it is recommended that you have a side-intake fan.
This is used to provide an additional amount of coolair and help in the overall airflow within your system
(see the pic below)
http://img177.exs.cx/img177/6036/picture0232lm.jpg
the next pic(s) will further show you how your gonna want it to be moved throughout your system.
http://img230.exs.cx/img230/6587/picture0252ar.jpg
http://img230.exs.cx/img230/4571/picture180ia.jpg
http://img37.exs.cx/img37/8516/cooling1bm.jpg
Thank you for taking time to review this guide, and hopefully this will help you :)
As always you can post and I, or the helpful folks here will point you in the right direction. Good luck and happy building your killer rig. :)

Depends, on how much fans and how well your airflow is. I never had to use a filter nor dust out my system.. once in a blue moon but not regularly. Filter are used more for noise reduction than dust filtration although there are end users that use them for both purposes.